A sudden subsidence of the main line railway tracks close to the Johannesburg-Germiston boundary prompted an investigation into the causes and mechanism of the subsidence. The area affected has been undermined in the past, but the movement is unusual in that the mining is at considerable depth. Subsidence has even occurred where the mining depth is in excess of 250 m. Important services traverse the subsiding area, including the four main line tracks, the M2 motorway in a 4 m deep cutting, Main Reef Road and a 1,2 m water main.

This technical note deals with a full-scale field trial comparing conventional Portland cement concrete with a pfa Portland cement concrete. The results were very encouraging and cost comparisons indicated a saving.